Spatial–temporal changes in the degradation of marshes over the past 67 years

Agricultural reclamation is widely regarded as a primary cause of marshes degradation. However, the process of marshes degradation on different geomorphology has rarely explored, which fail to explain the marshes degradation driven by natural restrictions in detail. The information deficiency unable propounded the targeted suggestions for the sustainable management of marshes. According to the development of China, we quantified the degradation rate of marshes on different geomorphic types from 1954 to 2020 in a typical transect in the Sanjiang Plain. The results indicated that (1) A total of 1633.92 km2 of marshes reduced from 1954 to 2020. And 97% (1582.35 km2) of marshes were converted to crop cultivation. The process of marshes degradation had obvious historical stages characteristics. The marshes degradation rate showed a trend of increasing first and then decreasing. The most serious period was 1995–2005 (6.29%) which was approximately 35 times of the period of before the reform and opening up (1954–1976) a minimal shrunk period. (2) The background of geological tectonic decided the whole trends in marshes degradation process. The degradation occurred first and worst in the meco-scale recent slow ascent region, and then extended to substantially recent slow subsidence region and the small-amplitude recent slow ascent region. (3) Significant location characteristics of marshes degradation reflected in this research. The spatial location of marshes degradation on the sub-regions sequentially consisted of alluvial plain, lower terrace, high floodplain, micro-knoll, low floodplain, and depressions. (4) Most of the existing marshes of the sub-Sanjiang Plain distribution in the national reserves. This study provides important scientific information for restoration and conservation of marshes.


Study area
The study was conducted in a subset of Sanjiang Plain, called sub-Sanjiang Plain. This region is the center area of Sanjiang Plain in Heilongjiang Province, northeast China, which locates north of Wanda Mountain (Fig. 1). The Sanjiang Plain is a low alluvial plain formed by three rivers: Heilongjiang River, Songhua River, and Wusuli www.nature.com/scientificreports/ River. The total area is 16,073.7 km 2 . The climate is temperate continental with four distinct seasons, including 6 months of freezing conditions 34 . The mean annual precipitation is 603.8 mm and mainly concentrates from June to August. The long freezing periods and clayey soil prevent water from infiltrating to deep layer. The geological structure type is Tongjiang Depression. The low-lying geomorphologic pattern in the area has been shaped by the surface subsidence since the Quaternary. The geomorphology types mainly included riparian zone and terrace.
Besides, a small amount of residual hill scatter in the vast low plain and the micro-landform are well developed on the riparian zones. The densely covered river, low and even terrain, and poor drainage rise the water table level in flood season 35,36 . All these natural and geographical conditions promote the formation and development of the marshes. Furthermore, the study area is suitable for the growth of annual crops such as corn, soybean and rice, etc. And the main soil types are meadow soil, swamp soil and albic soils.

Data preparation and methods
Many previous studies have shown that wetlands change in China have obvious historical characteristics 16,27,[37][38][39][40] . Reform and opening up is an important turning point of China's development. China's policy formulation, economic, agricultural and environmental development and changes were closely related to the basic national policy of reform and opening up 41 . Therefore, this study is based on the large social background of reform and opening up. Considering the availability of data and the lag of policy implementation, the land-use/cover datasets (1:100,000) of the typical transect zone in 1954-2020 were chose. The datasets derived from Northeast Institute of Geography and Agroecology (NEIGAE), Chinese Academy of Sciences (CAS) (http:// www. iga. ac. cn) and USGS Global Visualization Viewer (https:// glovis. usgs. gov/) ( Table 1). More detailed time points and reasons were shown in Table 1. In addition, an auxiliary data of 1:500,000 geomorphic map complied by the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (https:// www. resdc. cn/ data. aspx? DATAID= 260).

Data preparation.
To meet the purpose of this study, according to the land-use/cover classification system (  Methods. The criterions of transect establishment. The transect is an important method to study the spatial differences, which has been widely used in the fields of geography and ecology [42][43][44] . The based-transect method has become an important research tool to explore the zonal variation law in International Geosphere-Biosphere Program (IGBP) 43 . In order to accurately explore the temporal-spatial process and characteristics of the marsh loss and degradation in the sub-Sanjiang Plain, the criterions of the establishment of the typical transect are as follows 45 : 1) The transect is essential to cover all geomorphic types of the distribution area of the marshes in the sub-Sanjiang Plain; 2) The transect is essential to cover the main factors of the formation of the marshes; 3) The transect is able to describe the characteristics of the marsh changes in the sub-Sanjiang Plain from 1954 to 2020; 4) The transect region is able to represent the land use patterns of sub-Sanjiang Plain.
The condition of typical transect. In this study, we selected a typical transect with the size of 1905 km 2 from the sub-Sanjiang Plain (Fig. 1). Four main rivers flow through the transect including Heilongjiang River, Nongjiang River, Bielahong River and Wusuli River. The established transect mainly depends on the following factors: 1) The geomorphic types of the transect include alluvial depression, middle bar, alluvial plain, micro-knoll, low floodplain, high floodplain and lower terrace etc. Figure 2, which cover the main geomorphic types of marsh distribution in the sub-Sanjiang Plain; 2) The transect zone is shaped by the Tongjiang Depression. The reasons for the formation of the marsh in the sub-Sanjiang Plain are covered in this transect zone, including low-lying terrain, poor surface drainage and soil water penetration, well developed micro-geomorphy, humid climate and the dense plain river; 3) From 1954 to 2020, the marsh area has decreased by 82% from 1806.7 to 245.7 km 2 , which indicates that there is a severe degradation of marshes; 4) The landscape pattern was changed from the dominated marsh to the dominated farmland during the study period, which is able to represent the changes of land use pattern in the sub-Sanjiang Plain. 5) There are two main nature reserves in the transect zone, which are Sanjiang Heilongjiang National Nature Reserve and Bacha Island Heilongjiang National Nature Reserve (Fig. 2).
Segmentation of transect zone. Although the general trend of the neotectonics on sub-Sanjiang plain was descending, it was accompanied by intermittent rising in the process of subsidence, which was mainly controlled by the north-east secondary faults 36 . And due to the surface subsidence since the Quaternary, the geological structure type of the typical transect zone is Tongjiang Depression 20 . The specific manifestations are the Hei-longjiang_Nongjiang substantially recent slow subsidence region, the Nongjiang_Bielahong River meco-scale recent slow ascent region and the Bielahong River_Wusuli River small-amplitude recent slow ascent region. Combination of the geological formation movement, the typical transect was divided into three regions named first-region, including Heilongjiang_Nongjiang Region (I), Nongjiang_Bielahong River Region (II) and Bielahong River_Wusuli River Region (III) to reveal the spatial characteristics of marsh degradation. Then each region was further segmented into some sub-regions with the reference to fluvial geomorphic types, such as river region, middle bar region, depression region, alluvial plain region, micro-knoll region, lower terrace region, low floodplain region, high floodplain region and low hill region (Fig. 2).
Quantification of wetland degradation. For well understanding the rate of regional marshes changes and their characteristics differences, the marshes dynamic degree was proposed 46 . Wetland dynamic degree can be determined by the land use dynamic degree 47 . The equation is as follows: where U is the marsh area (km 2 ), b represents the current year and a represents the past years, LC is the dynamic degree (annual shrinkage ratio) of marshes area in T years.
Statistical analysis. In this study we mapped the spatial pattern in typical transect and calculated the conversion among farmland and marshes by ArcGIS software. For a better understanding of the patterns and process in marshes degradation, we quantified both the area and annual shrinkage ratio of marshes (LC) in six stage (Table 3). Specifically, we gathered statistic and compared the spatial-temporal variable changes among the first-regions and sub-regions.

Results
Spatial-temporal characteristics of marsh degradation. As Fig. 3 shown that the landscape patterns in the typical transect zone have dramatically changed during the 67 years, the landscape matrix from marshes changed into farmland. As Fig. 4 shown that paddy appeared around 2000 and gradually increased eventually, became the main landscape type in sub-Sangjiang Plain. Before this, marshes were only reclaimed into dry farmland. A total of 1633.92 km 2 of marshes reduced from 1954 (1806.71 km 2 ) to 2020 (172.9 km 2 ). Meanwhile, there was an increase of 1582.35 km 2 (82%) of crop cultivation, of which dry farmland and paddy fields were 332km 2 (17%) and 1250.35 km 2 (65%) respectively. Additionally, the loss of marshes caused by agricultural encroachment were different in the first-region. Which firstly occurred in I region and then expanded to other two regions (Fig. 5). Compared to region I and III, the marsh degradation rate in region II was faster and worse.
In   Figure shows that the typical transect is divided into three regions named first-region, including Heilongjiang_Nongjiang Region (I), Nongjiang_Bielahong River Region (II) and Bielahong River_Wusuli River Region (III). Then each region is further segmented into some subregions with the reference to fluvial geomorphic types, such as river region, middle bar region, depression region, alluvial plain region, micro-knoll region, lower terrace region, low floodplain region, high floodplain region and low hill region. Table 3. Description of historical stage.

Stage name Time intervals
The www.nature.com/scientificreports/ Quantification of the marsh degradation was shown in Table 4. The annual shrinkage ratio was continuously increased throughout the whole period which was rose rapidly at a rate of more than 3 times of each history stage in 1954-2005, but started slowly in the period of deepening reform and opening up until 2020. The marsh degradation rate was more notable in the period of transformation reform and opening up stage (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005), which was approximately 35 times of the period of before the reform and opening up (1954-1976) a minimal shrunk period.
Dynamic change degree of marsh. The annual degradation rate of marshes in regions I, II and III (Table 5) was all fluctuation, however, the trends of each region has it's own characteristic. The annual degradation ration of marshes in regions I and II were approximately 4 times and 3 times of the rate respectively  However, the rate of region II in each observation stage was both higher than regions I and III. In terms of the highest value, the max annual degradation rate in region II was 8.56%, which was 1.3 and 1.5 times in regions I and III, respectively, although the annual degradation rate decreased in the subsequent period, it was always greater than the highest values of regions I and III; then the annual degradation rate in region I was fall to 1.77% in 2005-2015 which was the minimum of the contemporaneity, and rose to almost the highest rate of region I in the next 5 years. Simultaneously, the annual degradation rate of marsh in region III declined slightly after continue growing from 1954 to 2015. In fact, after the period of the deepening of reform and opening up, the average annual degradation rate has remained high although it has changed.
Spatial location characteristics of marsh change. Heilongjiang_Nongjiang region. As with Fig. 6 and Table 6 shown, the marshes in District Heilongjiang_Nongjiang region were mainly distributed on high floodplains (202.7333 km 2 ), low floodplains (262.992 km 2 ), lower terrace (123.456 km 2 ), and depression (7.392 km 2 ). The sporadic degradation of marshes first occurred on the high floodplain , and then appeared in the lower terrace (1976)(1977)(1978)(1979)(1980)(1981)(1982). When the agriculture reclamation was extended to the floodplain (1990-1995), the marshes on depressions were also began to be continuously cultivated. However, after only 10 years, the marshes on depressions disappeared. It was worth mentioning that before 2005, the mashes were mainly converted into dry land but paddy fields after 2005. The intrusion of agriculture on various geographic was differed   The period of deepening reform and opening up 2015-2020 5.94 Table 5. The annual variance ratio of the marsh degradation based on the first level division. Nongjiang_Bielahong region. As Fig. 7 and Table 6 shown that marshes were mainly distributed on alluvial plains (299.916 km 2 ), lower terrace (256.839 km 2 ), followed by depressions (41.99 km 2 ), and only a few were distributed on low floodplains (5.65 km 2 ) and few micro-knoll (0.38 km 2 ). The reclamation of agriculture on the low floodplain was mainly happened 1982-2000 and the marshes on it were reduced by 89%; alluvial plain were 85% in 1954-2000; lower terraces were 89% in 1976-2005; and depression were 89% in 1990-2005. By 2020, the marshes distributed on the Nongjiang-Bielahong river region were less than 10 km 2 , almostly disappeared. The degradation process of marshes was different among different geographic. The Fig. 7 showed that the reclamation of farmland first started with large-scale reclamation on alluvial plain and lower terrace , then sporadic reclamation on low floodplain (1976)(1977)(1978)(1979)(1980)(1981)(1982), followed by depressions, finally micro-knoll (1985)(1986)(1987)(1988)(1989)(1990).
With the minimum area of marshes (0.378 km 2 ), micro-knoll occupied by farmland later but faster than other geographic and completely converted into farmland during 2000-2005. Compared with other geomorphic types, the process of marshes on depression eroded by farmland is the slowest, which was completely occupied by farmland until 2020.  www.nature.com/scientificreports/ Bielahong _Wusuli region. The main geomorphic types of Bielahong River_Wusuli river region were alluvial plain, depressions, lower terrace, low floodplains, high floodplains, and micro-knoll. Sporadic farmland reclamation first occurred in alluvial plain, lower terrace, and high floodplain . After 1982, the sporadic reclamation began on micro-knoll and low floodplain which has spread to alluvial depression until 1985. During the entire study period , the marshes on the high floodplain (29.585 km 2 ) in wetland reserves were almost uncultivated which only decreased by 12%. Unlike the overall shrinking trend in the region, the marshes distributed on the micro-knoll (24.531 km 2 ) were quickly reclaimed after 1985, only 2% of the initial area was left until 2005 and completely disappear in 2020. The marshes distributed on alluvial plain (254.557 km 2 ) was the largest which was degraded most seriously. Since 1985, large-scale and rapid farmland opening had occurred on alluvial plain, and the result showed that the marshes in this area were only 2.54 km 2 in the end. The area of marshes distributed on depressions and were 128.86 km 2 and 104.460 km 2 , respectively. The time node of lower terraces was basically the same as alluvial plain which was only 3% were retained. The marshes on depression had been reclaimed with highly speed after 2000, only 4% of the initial area were retained in 2020 (Fig. 8, Table 6).

Discussion
The historical stage of the change of marshes. In the period of before establishment of 'new' China, the sub-Sanjiang Plain had just experienced the ban policy of the Qing government, which had the sparsely populated and intact marshes accounting for 94% of the total area of the transect 48 . However, we found that only 0.09% of the total area of marshes in sub-Sangjiang Plain was remain by 2020, the landscape pattern of which had changed completely. Agricultural encroachment caused 86% of the total reduction in marshes in sun-Sangjiang  www.nature.com/scientificreports/ Plain from 1954 to 2020 (Fig. 9). Which was conformity with previous conclusions that agriculture activities are the most main driving cause of marshes 7,27,49 .
Throughout the shrinking process of the marshes in the sub-Sanjiang Plain, although the area of the marshes in the sub-Sanjiang Plain continued to decrease from 1954 to 2020, the whole process had obvious historical stages characteristic (Fig. 3, Table 4). The historical stage characteristics of marshes degradation in the sub-Sanjiang Plain reflect the national government's land policy, demand, and scientific and technological progress after the founding of the People's Republic of China 29 .
Before 2005, the annual degradation rate of marshes in sub-Sangjiang Plain continued increase. The annual degradation rate of marshes from only 0.2% in the reform and opening up (1954-1976) to 6.29% in the period of transformation reform and opening up (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005), increased 35 times approximately. But which was not a uniform growth and had its own historical characteristics. The "Great Leap Forward" (1958)(1959)(1960) and "The Down to the Countryside Movement" (1970)(1971)(1972) activities, made the marshes begun to be reclaimed 27,50 by a large number of immigrants poured into the Sanjiang Plain. Then the land reclamation demand of "Modern Farm Construction" (1979)(1980)(1981)(1982)(1983) and the stimulation of the household responsibility system 51 both promote the enthusiasm for reclamation. However, due to the limitation of technical conditions the degree of degradation was not serious at begin. With the support of the "Japan Loan Project" many large farming machinery and equipment had been put into use the rate and area of reclamation rose rapidly 52 , the rate of reclamation began to rise rapidly. Why the annual degradation rate in the period of transformation reform and opening up (1995-2005) reached 6.29%, the biggest? That's because of nearly 40 years of water conservancy project construction, Sangjiang Plain had initially formed three major engineering systems for flood control, waterlogging removal, and irrigation 53 . Good water conservancy and technology conditions provided the greatest convenience for the rapid development of agriculture.
Although the annual degradation rate of marshes was still high, it had begun to decline after 2005. That's all because of the never slackened protection of wetlands since China joined the "Convention on Wetlands of International Importance, Especially as Waterfowl Habitats" in 1992. In 2002, China implemented the "National Wetland Conservation Plan" (NWCP) and issued two plans for the pre-wetland protection project. In 2014, the red line of the wetland nature reserve was delineated. A series of policies about protection wetlands achieved significant effects in the protection and restoration, especially the wetlands in the Sanjiang Plain 54 . in addition, a national plan to adjust agricultural plantation structures was promoted and implemented actively by the local government, large areas of dry land converted into higher economic benefits paddy fields, reducing the reclamation of marshes. However, the policies of direct agricultural subsidy on crops after 2004 and cancelling taxes on crops from 2006 might have also encouraged farmers to convert marshes to cropland to get more income 38,55,56 . Additionally, the establishment of the World Trade Organization (WTO) in 2001 expanded the grain trading market which also made grain prices rose 16 . Then serious illegal cultivation has occurred. In a word, the drive of benefits caused the loss of marshes and the continuous increase of farmland.
The spatial-temporal characteristics of the change of marshes. The statistical results of the first regions and sub-regions in geomorphology indicated that the degradation process of marshes in the sub-Sanjiang Plain showed obvious spatial location differences in different historical stages and the degree of reclaiming of marshes on different geomorphic was different. From 1954 to 2020, because of the geological tectonic, the marshes degradation first occurred in the meco-scale recent slow ascent region which was most serious region, and then expanded to the substantially recent slow subsidence region and the small-amplitude recent slow ascent region. It was worth to note that the marshes degradation in the small-amplitude recent slow ascent region was still a trend to continue to expand. In different geomorphic regions, the degradation of marshes first began in alluvial plain, gradually spread to lower terrace and high floodplain, then followed to micro-knoll and low floodplain, and finally extended to depressions. This result fully reflected the restriction on farmland recla- www.nature.com/scientificreports/ mation by geomorphic conditions, reflecting the evolution of the man-land interrelations in the sub-Sanjiang Plain during 1954-2020. Before the period of comprehensive reform and opening up (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995), marshes reclamation mainly relied on manpower and nature conditions while being restricted by nature in the period of relative backward time which lack of machinery and equipment. Therefore, the reclamation of farmland first occurred in places with superior natural conditions, convenient transportation and easy access, and high population density. And the expansion of reclamation was centered on newly-built residential areas 55 .
As Figs. 3 and 5 shows that both region I and III were sporadic reclamation, continuous reclamation only in region II, and all occurred on the more accessible geomorphic alluvial plains, lower terrace and floodplains ( Table 6). Since then, the hydrological cycle of marshes had been changed because of the technology and productivity improved, reconstruction of wells, channels and well irrigation marshes project and continuous farmland reclamation which caused the edges of marshes continue to retreat to the depths of the marshes, and the boundaries of farmland continued to invaded. Conversions between marshes and farmland set up a positive feedback system in the marshes which transforms the marshes into upland, suitable for agricultural operations, and increase of agricultural operations further transforms parts of the marshes 25 . So far, human beings have moved from the stage of being greatly restricted by nature into a process in which humans can vigorously transform nature.
The reclamation sites of marshes had expanded from relatively flat alluvial plains and lower terrace to unsuitable micro-knoll and depressions. The distribution of farmland has also begun to deviate from residential areas and expand linearly along ditches and rivers. In the period of deepening reform and opening up (2005-2020), due to the insufficient reserve resources and implementing national protective measures, the degradation process of marshes had been suppressed to a certain extent, then the man-land interrelations in the sub-Sanjiang Plain had begun to develop in a good direction.
Implications of natural wetland management. The degradation of marshes in sub-Sanjiang Plain had been controlled to a certain extent, however, the marshes in sub-Sanjiang Plain still urgently need targeted management measures to promote the sustainable development of marshes.
Our study found that most of the existing wetlands in the sub-Sanjiang Plain exist in the National Nature Reserve. Which shows that the establishment of nature reserves plays an important role in the protection of wetlands. Therefore, the conservation and rehabilitation of natural wetlands needs consistent attention from both the government and the public. First, increasing the effectiveness protected areas of wetlands are essential measures for wetlands management. Second, improve the supervision of government to decline the phenomenon of illegal reclamation. Third, a sound national regulatory law is needed to solve the problem of varied protection level in natural wetland conservations several caused by government departments jointly controlled.
Research results proved that the degradation degree and sequence of marshes distributed on different geomorphic types are different. So, we reasonably inferred that the wetlands restoration speed and ultimate degree of restoration with different geomorphological backgrounds will also be limited by its natural conditions. Therefore, the priority areas for restoration should thus focus on the wetlands lost recently such as such as degraded wetlands isolated from large areas (depressions and mcro-knoll), or areas surrounding important wetlands for biodiversity conservation and ecosystem services. As for areas that have been cultivated for a long time such as alluvial plains, considering the changes in food security and hydrological conditions, dry farmland can be transformed to green paddy fields to create artificial wetlands. Improving the living environment quality of waterfowl.
It is impossible to separate the wetlands from agriculture on the Sanjiang Plain. Consider only the protection of wetlands without considering the development of agriculture is unadvisable 57 . The serious problems faced by wetlands are not only direct erosion, but also the opposition between food security and wetland protection, water use conflict between wetland and agriculture, and the pollution caused by the intensive application of chemical fertilizers and pesticides 58,59 . A balanced model that maximizes the benefits of agricultural production and life while ensuring the sustainability of wetlands needs to be explored. For example, develop green leisure agriculture to reduce chemical pollution; change the flood irrigation method of paddy field to centralized irrigation to reduce the conflict between paddy field irrigation and wetland water use.

Conclusion
According to the historical phases, this study adopted the time series of marsh dynamic change datasets derived from the existing land use data and Landsat series remote sensing images which revealed the changes of land-use/ cover patterns in the sub-sanjiang Plain. Study showed that the main reason for the shrinkage of the marshes was the occupation of farmland and the process of marshes farming is the evolution process man-land interrelations in the sub-sanjiang Plain. The process of marshes farming in the sub-sanjiang Plain exhibited significant historical stages and location characteristics with the development of the China and policy orientation was an important factor in the transformation of the human-land interrelations.
The period of transformation reform and opening up (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) was the most severe period of marshes degradation in the sub-sanjiang Plain which was also the most intense period of human-land conflict. After 2005, the degradation of marshes was prevented in a certain extent due to various protection policies, measures, and the establishment of natural reserves of wetlands. Most of the existing wetlands are distributed in national natural reserves. The marsh degradation was determined by the general regional geomorphic types.
During the study period, the degradation first occurred in the meco-scale recent slow ascent region, and then extended to the substantially recent slow subsidence region and the small-amplitude recent slow ascent region. The process of marshes degradation sequentially consisted of firstly began in alluvial plain, gradually spread to lower terrace and high floodplain, then followed to micro-knoll and low floodplain, and finally extended to depressions. www.nature.com/scientificreports/

Data availability
The datasets used during the current study are available from the corresponding author on reasonable request.